Centrifugal casting of jewelry is a development and refinement of the very old lost-wax method of casting. The lost-wax process was used by the ancient Greeks and Chinese over two thousand years ago. In the 1930's the manufacturing jewelers in the United States discovered that they could use the lost-wax process for casting jewelery. This was made possible through the development of a means for rapidly duplicating a jewelry model in wax by means of a rubber mold.
To make duplicate wax patterns for lost-wax casting, a mold of the original metal model is needed. Rubber molds are used extensively by jewelry manufacturers for the mass production of jewelry because of the flexible and elastic nature of the rubber mold and because rubber molds can be used for many years to make thousands of intricate patterns.
The process of manufacturing the rubber mold is well known in the art. An aluminum mold frame is used, into which is placed unvulcanized rubber. The model from which a rubber mold is to be made can be brass, copper, silver, gold, platinum or any firm material with a melting point of approximately 150.degree. F. or more above the vulcanizing temperature (310.degree. F.) of the uncured rubber. A sprue rod, used to provide the hole in the rubber mold through which wax will be injected, is then soldered to the model. The model is placed inside the aluminum frame and packed with rubber by conventional means. The rubber is then vulcanized in the mold by a commercial electric vulcanizer. When the uncured rubber has been vulcanized, the entire frame is cooled and then the rubber mold is removed from the frame. The rubber mold is then cut in half at least a sufficient amount to enable the model to be removed and the rubber can also be cut by conventional and well known means to create an intricate design mold.
It is well known in the prior art to inject casting wax into the rubber mold by means of a wax injector or a centrifugal machine in order to obtain a wax pattern replica of the model. After the wax has been injected into the rubber mold, it is allowed to set for 2-3 minutes and then carefully removed.
The resulting wax pattern replica, when fully cooled, will be approximately 10 to 15 percent smaller than the original model made from metal. This variation in size is caused by a combination of factors: (1) the elasticity of the mold rubber itself which is packed and vulcanized under heat and pressure and then expands to an unpredictable and variable degree in all dimensions to fill the cavity when the original metal model is cut away and removed from the rubber to create the mold; (2) variations in mold clamping pressure and exactness of registration of mold parts, coupled with the differences in temperature and pressure at which the wax is repeatedly injected into the mold to form multiple wax patterns for production; (3) shrinkage of the wax itself during cooling; (4) distortion of the wax during handling to remove the wax pattern from the rubber mold. While it is possible to make the model oversize in order to partially compensate for shrinkage, it is impossible to produce a finished wax pattern which is absolutely precise when compared to the original metal model because of the unpredictability of these various factors.
A group of wax patterns can then be assembled on a master wax sprue to form a tree which can be surrounded with investment in order to form the final mold for casting metal such as gold. The investment can be a combination of plaster of paris and quartz, or it can be composed of cristobalite, gypsum, silicas, and modifying agents. The investment must possess the power to withstand the required high temperature to remove the wax and the higher temperature and pressure of the molten metal without cracking. The investment is prepared by traditional and well known methods and is placed in a bell jar and vacuumed to remove trapped air from the investment. The wax pattern tree is then surrounded with the investment. Wax is then eliminated (burned out) by heating the object in a furnace at 1300.degree. to 1400.degree. F.
Metal such as gold is then injected into the cavities formed in the investment when the wax patterns were burned out by well known centrifugal casting means. In one such method, a flask containing the formed investment is placed on the end of one arm of the machine and the nozzle of the crucible which holds the molten gold is placed into the sprue opening of the flask and the investment. The other end of the machine's arm has adjustable weights to balance the arm. The arm is rotated by use of an electric motor in order to develop the necessary centrifugal force which is outward or away from the center, and this force injects the molten gold into the investment cavity. All the gold in the crucible enters the flask on the first rotation, and the arm continues to spin so that the metal remains in the flask until it solidifies. After the gold has been cast, the flask is removed from the centrifugal casting machine and is permitted to cool.
The resulting casting will be approximately one percent (1%) dimensionally smaller than the wax pattern used to form the investment capacity. This is due to the combined factors of shrinkage of the investment compound and shrinkage during the solidification of the metal cast. Then the investment is broken away and the gold casting is finished and polished by conventional means.
While the above described method of centrifugal casting provides finished products of substantial beauty, the method does not permit the casting to be made within uniformly precise tolerances. In the manufacture of most items of jewelry, such exact precision is not required. However, if an item of jewelry is to be matched with a second item which has been stamped or die struck to exact measurements, the conventional method of lost wax centrifugal casting is inadequate. As discussed above, this is primarily due to the reduction in size of the wax pattern replica which cannot be precisely controlled by conventional means known in the above described prior art and in addition due to the shrinkage of the investment and the metal being cast.